1. Field of the Invention
The present invention relates to a method for detecting fine particles contained in a fluid, and particularly to a method for detecting fine particles in a fluid with the use of X-rays.
2. Description of the Related Art
Various measurement techniques such as a sample screening method, a sedimentation method, a sedimentation transmission method using optical transmission, a light or laser diffraction/scattering method including dynamic light scattering, a photon correlation spectroscopy, a light-shielding method, an electrical sensing zone method, an image analysis method (microscopic method), a chromatography method, a cascade impactor method and a specific surface area measurement method are known as measurement or count techniques for fine particles in a powder which is an aggregate of fine particles, the diameters of the fine particles and the like. These techniques are used as appropriate according to object items of measurement concerning a state of a powder sample of a measurement object, a type of a particle size such as an average diameter and a statistical diameter, a distribution of particle sizes and a geometry of the particles such as a shape. The light diffraction/scattering method and the light-shielding method continuously measure or count the number of fine particles or a distribution of particle diameters of the fine particles in a gas or a liquid fluid, by charging the gas or the fluid containing the fine particles into a flow cell which is a narrow flow channel.
The scattering method is a method of irradiating the fine particles in the flow channel such as the flow cell with a laser or light, and measuring scattered light coming from the fine particles. The relationship between the intensity of the scattered light originating in light and the particle diameter of the fine particles is known on the basis of the Mie scattering theory to be that generally when the particles are several microns or smaller, the intensity of the scattered light is proportional to the fifth or the sixth power of the particle diameter, and when the particles are several microns or larger, the intensity of the scattered light is approximately proportional to the square of the particle diameter.
The optical transmission method or the light-shielding method is a method of irradiating the fine particles in the flow channel with light or laser light and measuring the intensity of light which decreases by being shielded. The amount of light decreased by being shielded is generally proportional to the cross-sectional area of the particles, and accordingly is approximately proportional to the square of the particle diameter when the fine particles have a spherical shape. The particle diameter is calculated from the relationship between this amount of the light signal and the particle diameter.
In the scattering method, when the particle diameter of the fine particles becomes large, the relationship between the particle size and the intensity of scattered light becomes complicated, and the intensity of scattered light greatly varies according to the particle shape, which accordingly makes it difficult to accurately measure the particle size. In the light-shielding method, when the particle size becomes small, the amount of shielded light becomes small, and the measurement error of the fine particles becomes large. For this reason, regardless of the type of the fluid, when the particle diameter of the fine particles is several tens of microns or less, the scattering method is often used, and when the fine particles have such a comparatively large particle diameter as several tens of microns or larger, the light-shielding method is often used.
The scattering method and the light-shielding method can be easily developed at a low cost, and accordingly are widely used in academic and industrial fields as an optical particle counter with the use of light or laser light. Examples of the utility of the scattering method can include measurement of aerosols in the general atmosphere, measurement of suspended airborne particles in a clean room and measurement of impure particles in pure water. Examples of the utility of the light-shielding method can include measurement of foreign matter particles having comparatively large particle sizes in liquids such as industrial water which mainly contains mud and industrial liquids like general drainage and lubricating oil. A technique is also widely used which measures a particle concentration or a distribution of particle diameters from the intensity of the whole scattered or transmitted light coming from the fluid without measuring individual particles. Furthermore, there is also a technique of measuring the image of the scattered light or transmitted light, and individually measuring the particles through image processing.
In addition, a large number of techniques have been developed which select or accurately measure the fine particles in a liquid with the use of the light-scattering method. A method has also been developed that measures optically-induced fluorescent light emitted from fine particles, which is generated by irradiation with light or a laser, and identifies the composition of the substance of the fine particles, and the substance such as a fluorescent substance, viable particles, bacteria and pollen.
However, the light-scattering method and the light-shielding method which irradiate the fine particles with light or a laser have a problem of measuring air bubbles contained in the fluid as fine particles, when measuring the fine particles contained in a liquid fluid in a flow cell. Air bubbles in a liquid have a refractive index largely different from that of a liquid passing in the flow channel, and accordingly refract the light on the interface between the liquid and the air bubbles, which results in scattering of the light. For this reason, because the scattering method measures the intensity of scattered light, it results in regarding and measuring air bubbles as fine particles. Because air bubbles scatter the light and decrease the amount of light to show the same effect as that of light-shielding, the light-shielding method results in regarding and measuring air bubbles as fine particles, in a similar way to that in the scattering method. For this reason, when fine particles in the liquid are measured with the scattering method and the light-shielding method, there is a fatal problem that the number, the concentration and the distribution of particle diameters of the fine particles cannot be accurately measured and errant measured data is shown. In addition, when more air bubbles exist in the liquid than the concentration of the fine particles, the effect of the light-scattering or the light-shielding of the light due to the air bubbles becomes remarkable, the effect of the light-scattering and the light-shielding due to the fine particles is strongly interfered by the effect of the light-scattering and the light-shielding due to the air bubbles, and as a result the measurement of fine particles is impossible. Accordingly, in order to suppress the formation of air bubbles in the liquid, separate the air bubbles from the fine particles, or identify and measure only the fine particles, techniques have been developed such as a technique of defoaming by heating, cooling and pressurizing the liquid, a technique of separating the air bubbles with ultrasonic waves, electrophoresis or the like, a technique of separating the air bubbles by a structure of a flow channel, and further a technique of identifying air bubbles by fluorescent light, a technique of removing air bubbles by capturing the air bubbles with a laser, and a technique of identifying air bubbles by analysis of the signal of the scattered light. However, it is difficult to completely separate or remove air bubbles even with these techniques of separating the air bubbles or defoaming, and a few fine air bubbles remain in the portion to be irradiated with the light or the laser in the flow channel. In order to discriminate air bubbles from fine particles as much as possible, and actually measure the fine particles in the liquid with high accuracy, it is necessary to use an ultrasonic wave, an electrophoretic force or a centrifugal force, to pressurize the liquid to a high pressure, to heat the liquid to an extremely high temperature or cool to an extremely low temperature, or to use each of these techniques in combination, which causes a great problem in developing the product of a measurement instrument that such an apparatus for separating the air bubbles becomes complicated and upsized, and further that a final cost concerning the measurement also increases.
In addition, when the fine particles in an oil product such as a lubricating oil are measured, there are also problems in that the optical transparency of incident light or laser light in the lubricating oil decreases due to coloring and the like caused by the deterioration of the lubricating oil, a normal irradiation intensity of the laser for the fine particles in the lubricating oil cannot be obtained because of the decay of the light by the lubricating oil, the intensity of the scattered light by the fine particles in the lubricating oil also decays, and accordingly the number and the particle diameter of the fine particles cannot be accurately measured. Furthermore, when the lubricating oil is contaminated with moisture content due to a operating environment and the like, micelles, which are droplets, are formed, in which one liquid substance is associated with the other in a granular form through emulsification (emulsion), and remarkably increases the scattering of light. Then, the light cannot pass through the liquid, and the measurement of fine particles becomes impossible.
Moreover, in the scattering method and the light-shielding method, when the fine particles to be measured are constituted by the same substance, a correlation is obtained between the intensity of the light signal obtained by the light-scattering or light-shielding and the particle size; but when the fine particles are made from different elemental compositions, the correlation cannot be obtained between the intensity of the signal and the particle size, because the fine particles made from different elemental compositions show different refractive indexes, characteristics of scattering the light, absorbing the light and transmitting the light and the like, which are optical characteristics of the fine particles; and the number of the particles and the particle diameter cannot be accurately measured. In the scattering method, when the fine particles are made from a substance which scatters little light or a substance which absorbs light greatly even though the particle diameter is the same, the intensity of the light signal of the scattered light coming from the fine particles becomes extremely small, and the signal is not counted as a signal, which consequently causes a count loss. Furthermore, even if the fine particles are made from the same substance, the scattering characteristics of the light are different between the case in which the surface shape of the fine particles is uneven and is complicated and the case in which the surface is smooth, and accordingly the particle diameter cannot be exactly measured. In the light-shielding method, even if the fine particles have the same particle diameter, when the substance of the fine particles absorbs little light and has a large transmittance for the light, a light-shielding effect due to the fine particles is small, and the fine particles are not counted as particles similarly to the case in the scattering method, which results in causing a count loss.
A method of detecting fluorescent X-rays which are emitted from an element due to irradiation with X-rays is proposed as a technique of identifying and detecting every element of the fine particles in a fluid (Japanese Patent Laid-Open No. 11-83767 and Japanese Patent Laid-Open No. 2006-29921).
Japanese Patent Laid-Open No. 11-83767 describes a concentration-measuring device for detecting a small amount of Fe and Cu which are substances of an object to be detected in a secondary supplied water in a pressurized light water reactor in a nuclear power plant. Japanese Patent Laid-Open No. 11-83767 discloses a method of detecting fluorescent X-rays which are emitted from a substance of an object to be detected, by providing a filter in a flow cell, collecting the substance of the object to be detected with the filter and irradiating the substance with X-rays.
Japanese Patent Laid-Open No. 2006-29921 discloses a flow-cytometer system of slowly passing a solution containing a sample (cell) through a tube, irradiating the sample with monochrome X-rays of high luminance from a monochrome X-ray irradiation system, and detecting fluorescent X-rays which are emitted from each element in the sample.
However, the technologies described in Japanese Patent Laid-Open No. 11-83767 and Japanese Patent Laid-Open No. 2006-29921 are both directed at detecting a specific element, are restricted to detecting the specific spectrum of the fluorescent X-rays, and accordingly cannot detect all fine particles contained in the fluid.
An object of the present invention is to provide a method and an apparatus which solve problems such as a measurement error due to air bubbles in a scattering method and a light-shielding method, a count loss due to a different element and impossible measurement due to emulsification, and can easily and accurately measure the number, the particle size and the like of fine particles in a fluid, at a low cost.